For localizing the origin(s) of cardiac arrhythmias it is common practice to measure the electric potentials located on the inner surface of the heart by electrophysiological means within the patient's heart. One method is to insert electrode catheters into the heart to record cardiac potentials during normal heart rhythm or cardiac arrhythmia. If the arrhythmia has a regular activation sequence, the timing of the electric activation measured in voltages at the site of the electrode can be accumulated when moving the electrode around during the arrhythmia, to create a three-dimensional map of the electric activation. By doing this, information on the localization of the source of arrhythmia(s) and mechanisms, i.e., re-entry circuits, can be diagnosed to initiate or guide treatment (radiofrequency ablation). The information can also be used to guide the treatment of cardiac resynchronization, in which implantable pacing electrodes are placed in specific locations within the heart wall or chambers to re-establish a normal level of coordinated activation of the heart.
A method using external sensors measures the electrical activity of the heart from the body surface using electrocardiographic techniques that include, for example, electrocardiograms (ECG) and vectorcardiography (VCG). These external sensor techniques can be limited in their ability to provide information and/or data on regional electrocardiac activity. These methods can also fail to localize bioelectric events in the heart.
A method using external sensors for the localization of cardiac arrhythmias utilizes body surface mapping. In this technique, multiple electrodes are attached to the entire surface of the thorax and the information of the cardiac electrograms (surface ECG) is measured in voltages that are accumulated into maps of cardiac activation. This measurement can be problematic because the electrical activity is time dependent and spatially distributed throughout the myocardium and also fails to localize bioelectric events in the heart. Complex mathematical methods are required to determine the electric activation upon the outer surface of a heart model (i.e. epicardium), for instance, one obtained from CT or MRI imaging giving information on cardiac size and orientation within the thoracic cavity.
Alternatively, recordings of potentials at locations on the torso, for example, can provide body surface potential maps (BSPMs) over the torso surface. Although the BSPMs can indicate regional cardiac electrical activity in a manner that can be different from conventional ECG techniques, these BSPM techniques generally provide a comparatively low resolution, smoothed projection of cardiac electrical activity that does not facilitate visual detection or identification of cardiac event locations (e.g., sites of initiation of cardiac arrhythmias) and/or details of regional activity (e.g., number and location of arrythmogenic foci in the heart).
Since the localization of cardiac arrhythmias by the use of potentials is imprecise, the successful treatment of cardiac arrhythmias has been difficult and has demonstrated limited success and reliability. There is, therefore, a need for improved methods of localizing, diagnosing and treating cardiac arrhythmias.